1. Field of the Invention
The present invention concerns a magnetic resonance (MR) imaging method with which additional image information (and thus more homogeneous MR images) can be acquired from an examined measurement subject. It also concerns CP (circularly polarized) coils or coil arrays formed by CP elements as well as a magnetic resonance tomography apparatus includes such CP coils or coil arrays for implementation of an MR imaging method.
2. Description of the Prior Art
MRT (magnetic resonance tomography) is based on the excitation of nuclear spins in body tissues and body fluids of a patient with pulses of radio-frequency energy as well as the detection of the RF signal emitted by the precessing of the nuclear spins. Data about the body tissue and body fluid are collected on the basis of this method, the data subsequently being processed into magnetic resonance images for medical diagnostics.
The quality (and thus significance) of the MR images is influenced by many factors. Examples of such factors are the homogeneity and strength of the basic field B0, the quality of the gradient coils (and therewith the gradient fields switched for slice selection and spatial coding), the filling factor and the performance of the transmission coils and reception coils of the RF system, the generation of auxiliary magnetic fields with shim coils in order to correct inhomogeneities of the basic field, etc. Deficiencies in the system can cause a number of image artifacts. Great efforts are therefore taken to optimize these factors and thus to continuously improve the quality of the generated MR images.
In addition to the above hardware aspects, a further possibility for influencing the quality and the information content of MR images is to vary or optimize the pulse sequence, meaning the chronological sequence of RF pulses, gradient switchings and signal detection. The development of new spin echo sequences such as the HASTE sequence and new gradient echo sequences such as the TrueFISP sequence has enabled the examination of body regions using MR technology that were not previously able to be imaged. This is particularly based on the fact that the T1 and T2 weighting (and therewith the contrast of the acquired MR images) is changed by the newer pulse sequences. T1 is the spin-lattice relaxation constant, T2 is the spin-spin relaxation constant.
In the further development of known methods and the development of newer methods, it is primarily intended to increase the resolution of the images and/or to reduce the measurement time. It is thereby imperative to acquire the MR images with a sufficiently high signal-noise ratio (SNR).
One way to improve the SNR is to increase the strength of the homogeneous basic field B0. Stronger resonance signals are thereby obtained. This approach, however, has disadvantages associated therewith. With higher field strengths, the wavelength of the applied radio-frequency fields reaches the magnitude of the subject dimensions. Under such conditions the penetration behavior of the fields in dielectric and conductive media (for example tissue) leads to a non-homogeneous distribution of the excitation field and to an inhomogeneous distribution of the reception sensitivity. The former directly leads to spatial variations of contrast and brightness in MR images, the latter to an additional variation of the brightness. Moreover, the chemical shift of the signals arising from fat and from water increases, causing artifacts to arise in the MR images. Since, at higher basic field strengths, the tissue counteracts the penetrating RF fields with a greater resistance, the amplitude of the RF excitation pulses must be increased, causing the specific absorption rate SAR to increase. Due to the SAR limits established by national health authorities for protection of the patient, this leads to limitations on the application side.
Even when short measurement times are intended, the SNR can in principle be improved by lengthening the measurement time. Movement artifacts in the MR image, however, increase with longer measurement times; longer measurement times are frequently not tolerated by the patient since his or her residence time in the magnetic resonance tomograph is thereby extended.
To solve this problem, it must be attempted, by optimization of other available parameters, to improve the SNR for a basic field strength B0 and a measurement duration in which the disadvantages described above do not occur, or are insignificant.
One solution involving the radio-frequency system is to use surface coils for the signal detection in the acquisition branch of the RF system. Such coils are directly placed on the region of the measurement subject (such as the human body) to be measured. The signal intensity and the SNR are increased due to the small distance between the measurement subject and the coil. Such surface coils generally are formed by LP (linearly polarized) reception elements, the signals of which are detected and processed independently of one another.
A further solution is the usage of transmission or reception coils in a CP configuration. The basic physical effect on which such a configuration is based is that the precession movement of the magnetization ensues in a defined direction. In the excitation case the local B1 field must follow this precession movement for the maximum MR efficacy, thus be right circularly polarized. Such a right circularly polarized B1 field can be generated with a CP transmission coil. During the signal detection the magnetization further precesses in this defined direction and therefore likewise locally generates a right circularly polarized B1 field. This right circularly polarized B1 field can be detected with maximum SNR when a CP reception coil is used.
A CP transmission coil or a transmission array formed by CP elements has at least two separate transmission systems that are designated as a “0° system” and a “90° system”. Each of these transmission systems ideally generates in the examination volume a linearly polarized electromagnetic field that temporally oscillates with the MR frequency. The field vectors of both of these linearly polarized magnetic fields are oriented orthogonally to one another and perpendicularly to the external magnetic field. In the CP transmission configuration the field vector of the sum field (generated by the superimposition of the component fields) of the precession movement follows the magnetization, meaning that a circularly polarized radio-frequency electromagnetic field then predominates at the site of the magnetization. This is technically achieved by a phase shift of the excitation current flowing through the 90° system by π/2 in comparison to the 0° system. The maximum MR-effective excitation field can be generated in this manner for a predetermined transmission power (and therewith a defined SAR).
A CP reception coil or a reception array formed by CP elements analogously has at least two separate reception systems. During the signal detection, the magnetization precesses and locally generates a right circularly polarized magnetic field. The MR signals detected by the two sub-systems are therefore ideally shifted in phase by π/2 relative to one another. These signals are constructively added by phase shifting of one of the two signals by π/2 for maximization of the signal/noise ratio SNR.
It should be noted that in the transmission mode and in the reception mode the phase shift of both sub-systems of the antenna structure ensues with different algebraic sign. For a pure transmission antenna or a pure reception antenna, a phase shifter is sufficient for realization of the performance advantage and the SNR advantage. If an antenna is used in both transmission and reception modes, a 90° hybrid component is typically used that in each mode provides the optimal combination of the signals.
Consistent with the statements above, in the prior art circularly polarized (CP) is equated with right circularly polarized, abbreviated in the following with “RCP”. In contrast, left circularly polarized, abbreviated in the following with “LCP” magnetic fields, are not used in MR imaging in the prior art.